1. Field of the Invention
The present invention relates to a radio receiving system based on an orthogonal modulation communication method, and more particularly, to a radio receiving system which compensates for an aperture effect due to a sampling operation by setting the frequency characteristics of a band pass filter—which permits passage of only a signal at a frequency band assigned to a communications system from which the radio receiving system receives a signal—are set so as to prevent the aperture effect.
2. Description of the Related Art
A receiver—which is based on a direct conversion receiving method and which has a simplified radio section—is realized through use of a channel filter which samples, or subjects to an analog-to-digital conversion operation, an input signal while the input signal still remains in a high frequency state before converted into a baseband signal and which subjects the quantized signal to a stable digital signal processing operation having a high degree of accuracy. However, the channel filter suffers from the following four problems.
First, as a result of the sampling operation, the sampling frequency renders the frequency characteristics of the overall radio receiving system uneven. Consequently, a digitized signal is demodulated at a high error rate.
Second, in order to highly accurately perform a sampling operation, previous and subsequent stages of the sampling circuit must have high speed characteristics required to ensure over a considerably wide frequency range the speed performance of the sampling circuit for the purpose of preventing the aperture effect. As a result, the sampling circuit has a bandwidth which is considerably wider than the bandwidth of a received signal. In short, in spite of a band pass filter provided in a previous stage in order to limit the bandwidth of the received signal to a predetermined bandwidth, the circuit provided in the subsequent stage must have a bandwidth which is significantly wider than that of the band pass filter. Thermal noise caused by the circuit provided in the subsequent stage exceeds the amount of that caused in an existing radio receiving system, which also accounts for an increase in the error rate.
Third, under the direct conversion receiving method, there is a need to provide a base band circuit with a function as a substitute for a channel filter which is conventionally provided in an IF stage of the existing receiver. To this end, it is also necessary for an HF stage whose filtering is insufficient to maintain a wide dynamic range and a wide bandwidth. Still further, there is a need for a filter which filters a signal having such a wide dynamic range and a bandwidth.
Fourth, a sampled signal usually includes d.c. components. Since the signal becomes vulnerable to d.c. noise, drift, or offsets, the signal including such noise accounts for a large error rate in the case of a portable cellular phone based on digital modulation.
FIG. 9 shows an example of an existing direct conversion receiver which uses a bandwidth-limited sampling method. This circuit diagram corresponds to a direct IF sampling circuit used in a new produce “125 MSPS Monolithic Sampling Amplifier AD9101” described in “Analog Devises Converter Data Book,” 1st edition, Analog Devises Co., Ltd., July, 1997. There are descriptions which state “Adoption of the Nyquist theory enables elimination of an IF frequency and reconstruction of a base band signal. For example, a 40-MHz IF signal is modulated by a signal having a bandwidth of 10 MHz, and a signal to be detected is detected at a sampling rate of 25 MSPS.” A 40-MHz IF signal modulated by a signal having a bandwidth of 10 MHz is usually detected at a sampling frequency which is twice as high as a frequency of 40 MHz. However, since the signal is limited to a bandwidth of 10 MHz, the IF signal can be detected at a sampling frequency of 25 MHz by utilization of the “Shannon's Sampling Theorem” according to which the IF signal can be sampled at a frequency twice or more as high as a frequency of 10 MHz.
FIGS. 10A to 10D are views showing variations in spectral components when direct conversion reception is performed through a bandwidth-limited sampling operation. FIG. 10A shows a desired waveform and adjacent waveforms in a radio frequency band, as well as the characteristics of a band pass filter which covers these waveforms. In the drawing, fs designates a sampling frequency set to a frequency which is twice or more as wide as a communications bandwidth or a the bandwidth of a bandwidth-limited filter.
FIG. 10B shows spectral components of the desired waveform and adjacent waveforms having frequencies converted into a baseband frequency at a sampling frequency. The baseband frequency range fBB is in principle the same as fBW.
FIG. 10C shows the result of extraction of the desired signal through a channel filtering operation, wherein a quantized signal obtained as a result of a sampling operation is subjected to a digital signal processing operation.
FIG. 10D shows the aperture effect caused by the sampling operation at this time. In other words, the drawing shows spectral components of what-is-called a sampling function. The spectral components have characteristics of {sin(πf/fs)}/(πf/fs), and a null occurs at a sampling frequency fs. Although the desired waveform in the range less than half the sampling frequency does not occur at the null point, the waveform is given the frequency characteristics which gradually attenuates a waveform toward higher frequencies.
The present invention relates to a receiver circuit having a built-in channel filter which is used with a radio receiving system assigned an offset frequency (disclosed in Japanese Patent Application Laid-open No. Hei-9-266452 “Receiving System” and Japanese Patent Application No. Hei-9-28271 “Receiving System,” both being filed by the applicant of the present patent application) and includes a complex coefficient filter. In the channel filter including a complex coefficient filter on which the present invention is based, the center of positive and negative frequency components to be subjected to a complex operation does not necessarily occur at zero frequency. For this reason, the aperture characteristics of the channel filter which cause the center of frequency components to occur at zero frequency make the operation distorted, resulting in a considerable decrease in the accuracy of the operation. Further, even if frequency components are shaped so as to have complete Nyquist characteristics through use of a subsequent root Nyquist filter, the frequency components cannot have the complete Nyquist characteristics.